Being skeptical that another person can be incapacitated by a mental illness because you cope just fine with your problems is basically the same as saying “I don’t understand why other people’s brakes fail, because my car works great.”
Just let them go. You’d be doing yourself a favor in the long run. It makes them feel better to feed off of you, knowing you always let them put you down. Whether it’s family or a childhood friend, you have to let go of the toxic people in your life. They’ll only cause more damage. Do what’s right for you, before you wish you done it sooner.